The present invention relates to a unitary, variable orifice pulse valve, for providing small pulsed volumes of gas. The valve may be used to supply a small dose of therapeutic gas, such as NO, or diagnostic gas, such as SF.sub.6, into the breathing circuit for a patient. The valve has a reduced internal volume to achieve accurate measurement of pulse volume. The present invention also relates to a special gas dose delivery unit and respiration equipment incorporating such a valve.
U.S. patent application Ser. No. 08/841,466 filed Apr. 22, 1997 by the present applicant describes an apparatus and method for the pulsatile dosing of a specified, special gas into a patient airway. The dose volume is defined by the dose pulse length and the flow occurring during the pulse. The volumes to be delivered may range from 10 .mu.l up to 20 ml. The pulse lengths may vary from 100 ms up to 1 s, and the flows from 0.05 ml/s to 20 ml/s. Proportionally adjustable valves can accommodate this flow range.
A problem with these valves is, however, in their dynamic behavior. They may not be sufficiently rapid, particularly when considering the smallest flows, where the fastest response is needed. Thus the system may have difficulties in dosing gas pulses of the smallest volumes.
In current state of the art, the dynamic range problem has been dealt with by diluting the special gas to enlarge the actual dose volumes to be delivered. European patent document EP 659,445 discloses a nitric oxide (NO) delivery system where this dilution can be done on "on fly" when the "system is otherwise unable to reduce the concentration to the desired point." The technique employs an additional diluting gas (nitrogen) source and a diluted NO concentration sensor for controlling the dilution. The disadvantages of this system are the increased complexity, size, weight, and cost caused by the diluting apparatus and instrumentation. Alternatively, the dilution may take place by selecting a tank of an appropriate concentration for the current need. A range of tank concentrations are presented, e.g. in U.S. Pat. No. 5,531,218 and European patent document EP 640,357. If the concentration in the tank is inappropriate for the treatment of the patient, it may be necessary to change tanks. Cumbersome maneuvers may be required when a need to change the concentration arises during the treatment of the patient.
A technique for creating small pulses is presented in patent document WO 95/10315. The delivery system shown in the document has a constant 12 l/min flow source and an on/off switch valve to create pulses. This kind of construction is cost-effective and simple. The low limit for the doses is set by the valve control time resolution. The system can provide pulses with a minimum length of 5 ms, corresponding to 1 ml of volume. However, even this volume is large compared to portions of the specified pulse volume range noted above. Furthermore, user control over the pulse amplitude and length are lost when pulse length is employed for dose control and the flow rate is fixed. Pulse width control is needed when a need to spread out the dose over a definite period of inspiration exists. The flow control may be beneficial in adjusting the local effectiveness of the treatment.
For safety reasons, a special gas dose delivery system should have two redundant means to prevent unintentional dosing of the patient. The apparatus described in applicant's earlier application and in European patent document EP 659,445 use a proportionally adjustable solenoid valve for the flow regulation, whereas U.S. Pat. No. 5,561,218 shows a control valve which could also be a proportional valve, but the disclosure of the patent does not have a detailed description. All of these systems do have the safety redundancy in the form of an on-off type solenoid valve in addition to the proportional valve. A fast response time is characteristic of an on-off solenoid valve.
The proportional adjustability of the proportional valve and the fast dynamic behavior of an on-off valve suggest the use of the proportional valve as a variable orifice and the on-off valve for delivery of the doses of gas.
However, a problem in such an arrangement comes from the volume existing between the proportional valve and the on-off valve. This volume comprises the internal volume of the valves and the intermediate channel volume between the valves. Due to the construction of such valves, the intermediate channel volume is not likely to be less than 20 .mu.l. The internal volumes of the valves depend on the valve type, but for commercially available miniature valves, it starts from about 40 .mu.l. These volumes, which together comprise easily 100 .mu.l, represent a pressure chamber for an uncontrollable dose of gas. The quantity of such a dose is dependent on the pressure chamber volume and the chamber pressure. As a result of the foregoing, the dose volume becomes uncontrollable when using the proportional valve as a variable orifice and the on-off valve for delivery of the doses.
The flow measurement of the dose has to be carried out upstream of the on-off valve to avoid the pumping effect on the flow measurement caused by the variable pressure conditions in the breathing circuit. The proportional valve acts as a flow restrictor. The more it restricts the flow the less the volume of the pulse. Downstream of the proportional valve are the uncontrollable dose volume and the on-off valve. Upon opening of the on-off valve, the pressure existing in the intermediate channel volume discharges and the pressure rapidly equilibrates with the pressure downstream of the on-off valve, which is practically near ambient pressure. After closing the on-off valve, the intermediate chamber will be reloaded with gas and re-subjected to the pressure of the gas. This re-application of the pressure takes place slowly through the orifice of the proportional valve. The reload volume can be detected and measured by the dose flow sensors, but only after the dosing has taken place.
At sea level ambient pressure, the upstream pressure of the delivery system is preferably 90 kPa overpressure or more, e.g. 150 kPa, to guarantee sonic flow in the discharge orifice when the valve opens for gas flow. The advantage of such a flow is the elimination of the effect of the variable pressure condition in the breathing circuit on the dose flow. With 100 kPa overpressure in the 100 .mu.l pressure chamber formed in the valves and intermediate channel, the uncontrollable dose will become 100 .mu.l by volume. This represents a volume tenfold greater than the minimum dose requirement noted, but could be even more depending on the valves used, and represents the minimum dose volume that can be delivered with such an arrangement.
Inverting the order of the proportional and on-off valves, to place the on-off valve upstream of the proportional valve would help in regulating the dose volume. However, the dose timing would be totally lost, the dose pulse "leaking" little by little through the orifice of the proportional valve over a relatively long time period.